You can do inplace editing of files:

But that introduces a race condition. The problem is documented in perlrun:

-i[extension]

specifies that files processed by the <> construct are to be edited in-place. It does this by renaming the input file, opening the output file by the original name, and selecting that output file as the default for print() statements.

The race: Instance 1 is started, reads the configuration file, then starts rewriting by renaming the configuration file and creating a new, still empty (or partly written) configuration file. Instance 2 is started, and reads an empty (or partly written) configuration file.

How to get rid of that problem:

• Use a (separate) lock file to synchronise access to the configuration file. Only the instance that can successfully lock the lock file may read or write the configuration file.
• Use an atomic operation to change the configuration file. The only way I know is to rewrite the configuration file to a new, temporary file, and to rename that file to the original configuration file name.

(Note that safely creating temporary files is another problem: mkstemp(3) and tmpfile(3) should be safe. mktemp(3) and tmpnam(3) aren't. File::Temp has safe and unsafe functions.)

Alexander

(And yes, the problem of "disappearing configuration data" has bitten me once, and it was quite hard to find.)